Cellulase can be used to modify the cellulase surface of cellulosic fibers enhancing the efficiency of mechanical refining of wood fiber saving refining energy in papermaking. While the combined action of the cellulase treatment followed by mechanical refining of cellulosic fiber helps in fibrillating the fiber, many commercial cellulases also contain specific cellulase activities that are capable of defibrillating cellulosic fiber by hydrolyzing the fibrillated area on the fiber surface. This action of cellulase is detrimental for paper dry strength properties as the fibrillated area is needed for better fiber to fiber interaction in a paper product upon drying and providing better dry strength. In addition, those specific cellulase activities mentioned above may be capable of hydrolyzing small cellulosic fiber debris or fine particles. While this property of cellulase can help reduce pulp viscosity and improve pulp drainage; it can also cause fiber loss with increased chemical oxygen demand (COD) in paper production. It is not mechanistically clear how a cellulase product can be applied to a papermaking process for improving dry strength properties of a paper product.
Cellulase is generally referred to as an enzyme composition derived from a microorganism fungi or bacteria that can catalyze the hydrolysis of β-1, 4-glycosidic bonds of a cellulase molecule or its derivatives. As shown in Table I, endo-cellulases, exo-cellulases and cellobiase cellulases are three types of specific cellulases that have distinctive activity that is different from each other towards specific cellulase molecules. The three types of cellulases are physically, chemically and enzymatically different. Among them, endo-cellulase or β-glucanase randomly hydrolyzes internal amorphous anomalies within crystalline cellulase, yielding high oligosaccharides or shortened cellulase polysaccharides. Exo-cellulases or exo-cellobiohydrolase (CBH1 or CBH2) release oligosaccharides of a degree of polymerization (DP) of 2 to 4 from the reducing end or non-reducing end of a cellulase polymer. Cellobiase or β-glucosidase has no activity towards cellulase polymer or oligosaccharides but catalyzes the hydrolysis of cellobiase to glucose. Cellulases are used in a variety of industries and are produced in large scale from various species such as Trichoderma, Humicola, Thermomyces, Bacillus, etc . . . via genetic enzyme engineering.
To determine endo-cellulase activity in a cellulase product, a water soluble cellulase derivative such as carboxymethyl cellulase (CMC) or hydroxyethyl cellulase (HEC) is conventionally used as a substrate and the reducing sugar released by the enzyme is measured by a dinitrosalicylic acid (DNS) method. The exo-cellulase activity may be distinguished from the endo-cellulase activity by using water insoluble cellulase such as cellulase filter paper or wood fiber as a substrate and the reducing sugar released from the insoluble fiber is then determined by the DNS method mentioned above. The cellobiase activity in a cellulase product is usually determined using cellobiose as a substrate, and the amount of glucose released is assayed using a glucose oxidase (GO) method.
TABLE IClassification of CellulaseEnzymeCellulaseNameNomenclatureEnzyme AssayEndo-β-GlucanaseE.C.3.2.1.4CMC or HEC as substrate, and use the DNSCellulasemethod to measure the reducing sugarcontentExo-Exo-E.C.3.2.1.91Cellulosic fiber as substrate, and use theCellulaseCellobiohydrolasesDNS method to determine reducing sugar(CBH1 and CBHII)releasedCellobiaseβ-GlucosidasesE.C.3.2.1.21Cellobiose as substrate, using the GlucoseOxidase method to determine glucosereleased
A cellulase derived from microorganisms may contain all three types of cellulases. While such a product can work synergistically to attack crystalline cellulase and convert it to small sugars, and eventually to glucose, it is not preferred for use in papermaking applications to improve paper dry strength. The endo-cellulase activity in the cellulase product attacks the amorphous anomalies within the crystalline cellulase and disrupts the crystalline structure. This enhances the efficiency of mechanical refining in fibrillating cellulosic fiber and helps improve dry strength of a paper. However, the exo-cellulase activity that exists in the cellulase product may defibrillate the cellulosic fiber and generate cellulase fines. In theory exo-cellulase activity may help improve pulp drainage via defibrillation, but it could also have a negative effect on paper dry strength properties. Not all cellulases are effective for paper strength applications and some can actually hurt the dry strength properties.
A cellulase derived from a microorganism may have multiple components with more than one endo-cellulase and exo-cellobiohydrolase. For example, a cellulase from Trichoderma longibrachiatum can have two CBH components, CBH I and CBH II, and three endo-cellulase components, EG I, EG II and EG III. A mono-component cellulase can be produced by the cloning of a specific cellulase DNA sequence encoding the single cellulase and expressed in a host organism. In other words, a mono-component endo-cellulase is a single endo-cellulase component essentially free of other cellulases such as exo-cellulases and β-glucosidase that usually exist in a cellulase product produced by a conventional microorganism. Single endo-cellulases can be used in the present invention for improving dry strength of a paper product in papermaking.
U.S. Pat. Nos. 5,169,497, 5,423,946, 6,770,170, 6,939,437, and U.S. Patent Appl. No. 20110168344, disclose that a cellulase product can be used to improve drainage of a wood pulp when used in combination with cationic polymers. However, the references are silent on how those combinations affect paper dry strength, which specific cellulases may be used in the application or how the cellulase dosage affects the performance for paper dry strength.
U.S. Pat. No. 5,507,914 (the '914 patent), describes a process for enhancing pulp freeness and also paper strength using a combination of a cellulase with a cationic polymer. The '914 patent teaches a dosage level of 0.05-0.25% cellulase based on the dry pulp was used. This is equivalent to about 2500 ECU/kg to about 12500 ECU/ kg dry fiber based on the present invention. Our studies indicate that at these higher addition levels, dry strength properties are negatively impacted.
U.S. Pat. No. 6,635,146 (the '146 patent), discloses a method of treating papermaking wood fibers using a one or more truncated hydrolytic enzyme in amounts of 5,000 ECU to 200,000 ECU per kilogram of fiber.
U.S. Patent Appl. No. 20020084046 (the '046 application), describes a process for making paper by adding an enzymatic material to a storing stage that is subsequent to the pulping or refining stage for a paper product having improved softness, bulk and absorbency while maintaining strength.
General literature teaches that cellulase activity may be improved in an enzyme assay when used in combination with anionic and non-ionic surfactants. The possible mechanism is that the surfactants reduce cellulase adsorption to non-cellulase components such as lignin, free cellulase for the cellulosic substrate and aid in thermal stability of the cellulase protein. Tween 20 and Tween 80 are two examples of such surfactants. Polyethylene glycol and its surfactant derivatives may also help improve cellulase activity in cellulase assays. However, little information is available in public on using combination of cellulase and surfactants in papermaking application and how those combinations would affect specific activities of the three different types of cellulases.
U.S. Patent Appl. No. 20040038841 discloses a cellulase formulation produced from nonionic surfactants together with endo-glucanases derived from Zygomycetes, which can be used in the treatment of fabrics.
Japanese Patent No. 5507615 discloses a polyvinyl alcohol and polyvinylpyrrolidone) in a cellulase formulation to enhance cellulase activity.
The publications listed above are all incorporated herein by reference.